1. Field of the Invention
The present invention generally relates to a drive unit, an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine including at least two of these functions, that includes the drive unit, and a driving method therefor.
2. Discussion of the Background Art
Generally, motors (i.e., drive sources) used in electrophotographic image forming apparatuses are required to rotate at multiple different velocities corresponding to the operational mode of the image forming apparatus, which in turn depends on image quality and recording media type. Accordingly, margin of allowable torque is dependent on the velocity. That is, when the velocity is lower, the margin is greater, thus increasing adverse effects such as heat generation or vibration. To avoid such adverse effects, several approaches described below have been tried.
For example, the electrical current for the motor may be adjusted to reduce the margin of allowable torque. More specifically, pulse-width modulation (PWM) control is used, or the channel is switched for each threshold of the electrical current. These approaches, however, have several drawbacks. For example, the capacity of the software required for the control and the number of control-related components increase. Consequently, the required space as well as the cost increases.
Alternatively, inrush electrical current may be controlled by resistors having multiple fixed resistances to reduce the margin of allowable torque. However, it is difficult to switch the fixed resistance on the driving source. Additionally, the number of control-related components, the required space, and the cost increase similarly to the first approach described above. Thus, it is difficult to provide a compact image forming apparatus at a reduced cost.
In view of the foregoing, for example, JP-2003-278441-A proposes a direct current (DC) motor that includes a low-velocity brush, a high-velocity brush, and a common brush, and a control circuit switches the brush between the low-velocity brush and the high-velocity brush depending on the velocity. The DC motor rotates at high velocity with a lower torque when the common brush and the high-velocity brush are activated and rotates at low velocity with a higher torque when the common brush and the low-velocity brush are activated.